This invention relates to metal fatigue testing and more particularly to testing which identifies the extent and location of fatigue damage accumulation prior to the initiation of fatigue cracks.
The early detection and assessment of the accumulation of fatigue damage in metal parts is an old and complex problem. A wide variety of methods have been explored but with very limited success. The objective has heretofore usually been to detect the initial fatigue crack. Such observations provide useful information but do not provide a means to estimate the severity of the damage in terms of the remaining portion of the useful fatigue life. I have earlier observed and reported (e.g., Metallurgical Transactions A, Vol. 8A, 899-904, June 1977) that an early manifestation of the accumulation of fatigue damage is the development of surface deformation at certain locations of the part during fatigue testing which locations eventually become sites for nucleation of fatigue cracks. I have further observed that long before fatigue cracks appear in the metal, the surface deformation produced microcracks in the natural layer of brittle surface oxide that forms on the metal and that the bare metal at the base of the microcracks could be located and quantified by scanning the specimen's surface with a small spot of ultraviolet radiation and measuring the region where intense exoelectron emission developed. While this process was effective, the procedure was complex, time consuming and required rather expensive and sophisticated equipment.